Lev N. Zakharov

Aqueous formation and manipulation of the iron-oxo Keggin ion

From iron clusters to iron mineral Growing a mineral out of solution, either in the lab or in nature, requires the assembly of atoms or clusters of ions. The structure of some common iron oxides hints that tiny iron-oxygen clusters may serve as mineral building blocks, but isolating these often unstable clusters is challenging. Sadeghi et al. not only isolated but were able to control the growth and dissolution of an iron-oxygen cluster that is a likely precursor to the most common iron oxide mineral, ferrihydrite. Science, this issue p. 1359 A 13-atom iron-oxygen cluster acts as a precursor to ferrihydrite crystallization. There is emerging evidence that growth of synthetic and natural phases occurs by the aggregation of prenucleation clusters, rather than classical atom-by-atom growth. Ferrihydrite, an iron oxyhydroxide mineral, is the common form of Fe3+ in soils and is also in the ferritin protein. We isolated a 10 angstrom discrete iron-oxo cluster (known as the Keggin ion, Fe13) that has the same structural features as ferrihydrite. The stabilization and manipulation of this highly reactive polyanion in water is controlled exclusively by its counterions. Upon dissolution of Fe13 in water with precipitation of its protecting Bi3+-counterions, it rapidly aggregates to ~22 angstrom spherical ferrihydrite nanoparticles. Fe13 may therefore also be a prenucleation cluster for ferrihydrite formation in natural systems, including by microbial and cellular processes.

Electrolytic synthesis of aqueous aluminum nanoclusters and in situ characterization by femtosecond Raman spectroscopy and computations

Significance The aqueous chemistry of Al has a large impact across the biosphere, hydrosphere, geosphere, and anthrosphere. Despite a century of study, aqueous Al chemistry and speciation are still not well understood because of the challenges of selectively isolating and synthesizing specific Al-containing aqueous clusters and of precisely characterizing those clusters. We report the atom- and step-economical electrolytic synthesis of aqueous Al clusters combined with improved femtosecond Raman spectroscopy and computations to elucidate the structures and formation pathways of aqueous clusters in situ. We demonstrate the unique power of this integrated platform by synthesizing and characterizing flat [Al13(μ3-OH)6(μ2-OH)18(H2O)24]15+ clusters in water, which are versatile precursors for large-scale preparation of Al2O3 thin films and nanoparticles for electronics, catalysis, and corrosion prevention. The selective synthesis and in situ characterization of aqueous Al-containing clusters is a long-standing challenge. We report a newly developed integrated platform that combines (i) a selective, atom-economical, step-economical, scalable synthesis of Al-containing nanoclusters in water via precision electrolysis with strict pH control and (ii) an improved femtosecond stimulated Raman spectroscopic method covering a broad spectral range of ca. 350–1,400 cm−1 with high sensitivity, aided by ab initio computations, to elucidate Al aqueous cluster structures and formation mechanisms in real time. Using this platform, a unique view of flat [Al13(μ3-OH)6(μ2-OH)18(H2O)24](NO3)15 nanocluster formation is observed in water, in which three distinct reaction stages are identified. The initial stage involves the formation of an [Al7(μ3-OH)6(μ2-OH)6(H2O)12]9+ cluster core as an important intermediate toward the flat Al13 aqueous cluster.

Investigation of Functionalized α‐Chloroalkyllithiums for a Stereospecific Reagent‐Controlled Homologation Approach to the Analgesic Alkaloid (−)‐Epibatidine

Four putative functionalized α-chloroakyllithiums RCH2CHLiCl, where R=CHCH2(18 a), CCH (18 b), CH2OBn (18 c), and CH[O(CH2)2O] (18 d), were generated in situ by sulfoxide-lithium exchange from α-chlorosulfoxides, and investigated for the stereospecific reagent-controlled homologation (StReCH) of phenethyl and 2-chloropyrid-5-yl (17) pinacol boronic esters. Deuterium labeling experiments revealed that α-chloroalkyllithiums are quenched by proton transfer from their α-chlorosulfoxide precursors and it was established that this effect compromises the yield of StReCH reactions. Use of α-deuterated α-chlorosulfoxides was discovered to ameliorate the problem by retarding the rate of acid-base chemistry between the carbenoid and its precursor. Carbenoids 18 a and 18 b showed poor StReCH efficacy, particularly the propargyl group bearing carbenoid 18 b, the instability of which was attributed to a facile 1,2-hydride shift. By contrast, 18 d, a carbenoid that benefits from a stabilizing interaction between O and Li atoms gave good StReCH yields. Boronate 17 was chain extended by carbenoids 18 a, 18 b, and 18 d in 16, 0, and 68% yield, respectively; α-deuterated isotopomers D-18 a and D-18 d gave yields of 33 and 79% for the same reaction. Double StReCH of 17 was pursued to target contiguous stereodiads appropriate for the total synthesis of (-)-epibatidine (15). One-pot double StReCH of boronate 17 by two exposures to (S)-D-18 a (≤66 % ee), followed by work-up with KOOH, gave the expected stereodiad product in 16% yield (d.r.~67:33). The comparable reaction using two exposures to (S)-D-18 d (≤90% ee) delivered the expected bisacetal containing stereodiad (R,R)-DD-48 in 40% yield (≥98% ee, d.r.=85:15). Double StReCH of 17 using (S)-D-18 d (≤90% ee) followed by (R)-D-18 d (≤90% ee) likewise gave (R,S)-DD-48 in 49% yield (≥97% ee, d.r.=79:21). (R,S)-DD-48 was converted to a dideuterated isotopomer of a synthetic intermediate in Coreys synthesis of 15.

Modification of Rifamycin Polyketide Backbone Leads to Improved Drug Activity against Rifampicin-resistant Mycobacterium tuberculosis

Background: The emergence of drug-resistant tuberculosis has called for the discovery of new antitubercular drugs. Results: We successfully generated 24-desmethylrifampicin by modifying the rifamycin polyketide backbone. Conclusion: 24-Desmethylrifamycin showed better antibacterial activity than rifampicin against multidrug-resistant strains of Mycobacterium tuberculosis. Significance: The combined genetic-synthetic strategy used in the study has opened up new avenues for generating more rifamycin analogs. Rifamycin B, a product of Amycolatopsis mediterranei S699, is the precursor of clinically used antibiotics that are effective against tuberculosis, leprosy, and AIDS-related mycobacterial infections. However, prolonged usage of these antibiotics has resulted in the emergence of rifamycin-resistant strains of Mycobacterium tuberculosis. As part of our effort to generate better analogs of rifamycin, we substituted the acyltransferase domain of module 6 of rifamycin polyketide synthase with that of module 2 of rapamycin polyketide synthase. The resulting mutants (rifAT6::rapAT2) of A. mediterranei S699 produced new rifamycin analogs, 24-desmethylrifamycin B and 24-desmethylrifamycin SV, which contained modification in the polyketide backbone. 24-Desmethylrifamycin B was then converted to 24-desmethylrifamycin S, whose structure was confirmed by MS, NMR, and X-ray crystallography. Subsequently, 24-desmethylrifamycin S was converted to 24-desmethylrifampicin, which showed excellent antibacterial activity against several rifampicin-resistant M. tuberculosis strains.

Solid-state dynamics of uranyl polyoxometalates.

Understanding fundamental uranyl polyoxometalate (POM) chemistry in solution and the solid state is the first step to defining its future role in the development of new actinide materials and separation processes that are vital to every step of the nuclear fuel cycle. Many solid-state geometries of uranyl POMs have been described, but we are only beginning to understand their chemical behavior, which thus far includes the role of templates in their self-assembly, and the dynamics of encapsulated species in solution. This study provides unprecedented detail into the exchange dynamics of the encapsulated species in the solid state through Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy. Although it was previously recognized that capsule-like molybdate and uranyl POMs exchange encapsulated species when dissolved in water, analogous exchange in the solid state has not been documented, or even considered. Here, we observe the extremely high rate of transport of Li(+) and aqua species across the uranyl shell in the solid state, a process that is affected by both temperature and pore blocking by larger species. These results highlight the untapped potential of emergent f-block element materials and vesicle-like POMs.

Spontaneous symmetry breaking during interrupted crystallization of an axially chiral amino acid derivative

High net enantiomeric excess was observed for crystal collections obtained by crystallization of the TFA salt of a configurationally stable yet racemic axially chiral amidoamine in EtOH solution with or without stirring (up to >99% ee at < or = 15% crystallization).

Cesium salts of niobo-tungstate isopolyanions with intermediate group V–group VI character

Alkali metal salts of polyoxometalates (POMs) of the group VI elements (W and Mo) and polycoltanates (Nb and Ta POMs) exhibit opposing trends in their solubility in water and ion-association in solution. Mixed clusters of these two group V metals and tungsten provide an opportunity to probe the reversal in these trends and to understand their origin. A review of a classic study of mixed Nb/W clusters and our own work in Ta/W polyanions have led us to isolate Cs+/Na+ salt of [Nb4W2O19]6− and two salts, Cs+/Na+ and pure Cs+, of [Nb2W4O19]4− by using peroxoniobate ([Nb(O2)4]3−) instead of hexaniobate ([Nb6O19]8−) as the niobium source. Crystallographic analysis shows that Cs+-bonding to clusters increases with Nb-content, following the trend observed in our previous studies of hexaniobate in solution. Fragmentation by ESI-MS suggests that niobium-rich [Nb4W2O19]6− is less stable than isostructural [Nb2W4O19]4− and this technique, together with FTIR, confirms the predominance of the cis-isomer in the cluster structures. The mixed-metal composition of these isopolyanions is reflected in the crystallographic bond lengths and in the positions of the absorption bands in the UV spectra. DFT calculations reveal that the HOMO–LUMO energy gap widens with increasing Nb content in the cluster framework – an effect ascribed to the overall poorer mixing of Nb4d, versus W5d, atomic orbitals with the corresponding O2p orbitals.

New A2/3−xRh2O4 Compounds with the CaFe2O4 Structure Where A Is a Rare Earth or Bi

New compounds of the type R(2/3-x)Rh(2)O(4) with the CaFe(2)O(4) structure have been prepared, where R is a rare earth. For crystals grown in a Bi/V/O flux, the rare earth was partially replaced by Bi. No evidence of ordering of the A cation vacancies is found, but the A cations are displaced from the ideal A cation site by about 0.24 A. Electrical conductivity measurements on crystals suggest that the materials are degenerate semiconductors with Seebeck measurements showing p-type behavior. This is consistent with our observation that x in R(2/3-x)Rh(2)O(4) ranges up to about 0.09. The compounds were also characterized by magnetic susceptibility and diffuse reflectance measurements.